1. Field of the Invention
The present invention relates to a residual current transformer for detecting earth leakage current etc. The present invention is to improve an overinput characteristic as one of the important main characteristics of a current transformer by using an annular or frame like magnetic core of the current transformer which has no gap nor sectional part crossing the magnetic passage but has at least one narrow sectional part in the magnetic passage.
The present invention is also to provide a residual current transformer having excellent characteristics of the improved over-input characteristics and small residual output of unbalanced characteristics by using a magnetic core having said narrow sectional area part and an annular shield made of a magnetic substance on a secondary winding wound on the magnetic core.
2. Description of the Prior Arts
Referring to FIGS. 1 and 2, the conventional residual current transformer will be illustrated.
In FIGS. 1 and 2, the reference numeral (1) designates a laminated magnetic core; (2) designates a box for holding a magnetic core; (4) designates the secondary winding of the current transformer; (5) designates an insulating tape or a box for holding the current transformer; (6) designates a filler made of a varnish or a synthetic resin; (7) designates an opening for passing the primary conductor; (8) designates a terminal wire of the secondary winding; and (10) designates the primary conductor; and (11) designates an insulating coat.
Such conventional current transformer comprises a slit remained in parallel to the opening surface in the inside of the annular magnetic core i.e. the opening side and the other part covered by the doughnut form magnetic shield covering the magnetic core. The other conventional current transformer comprises a flat annular magnetic plate at the side surface of the current transformer in parallel to the opening of the magnetic core. The other conventional current transformer comprises a magnetic shielding plate along the inner peripheral part or the outer peripheral part of the current transformer.
When alternating current having the same reverse currents are passed through the primary conductor (10) of the residual current transformer, the magnetizations of the magnetic core (1) surrounding the primary conductor are respectively cancelled to be substantially zero. When a current leak of a load is given, the cancellation of the magnetizations is not completed so as to cause a magnetization by the current corresponding to the unbalanced current i.e. the leak current, whereby an electromotive force corresponding to the leak current of the secondary winding is formed.
When the leak current having more than allowable limit is passed in the load side, the electromotive force of the secondary winding corresponding to the leak current (output) or its signal actuates the breaking mechanism to break the current in the primary conductor circuit in leak thereby preventing a damage.
The residual current transformer is a device for detecting such small current. The primary conductor can be a three phase AC circuit. The operation mechanism is the same as the former one. Thus, the residual current transformer should actuates a breaking mechanism in high reproducible, by the leak current having greater than the predetermined current, whereby it is necessary to have desirable AC secondary output, overinput characteristic and unbalanced characteristic (residual output characteristic).
In order to provide the desired AC secondary output, it is necessary to have higher permeability and smaller loss.
The overinput characteristics means the phenomenon that the output of the residual current transformer is relatively decreased to increase the working current after passing large leak current such as the earthing short-circuit of the residual current transformer. The overinput characteristic is given by dividing a deteriorated output after passing the large leak current by the output before passing the large leak current and shown as a percent. The specification limit of the characteristic is usually less than 10-15% and preferably smaller.
It is considered that the deterioration of sensitivity is usually depending upon the magnetization characteristic of the magnetic core (1).
FIG. 3 shows the magnetization characteristic of the magnetic core wherein the excitation magnetic field is plotted on the abscissa and the magnetization is plotted on the ordinate. The full lines of (10)-(8)-(11) show the magnetization curves. The loop (12) shows the magnetization curve corresponding to the magnetization by the normal alternating current whereas the line (9) shows the magnetization curve corresponding to the magnetization by the alternating current near the residual magnetization after passing the large excitation current. The reference numeral (8) designates the point of the magnetization of the magnetic core at the time passing the large excitation current; and (10) and (11) respectively designate coercive forces. When the residual magnetization (Br) after passing the large excitation current is higher, the position of the magnetization curve (9) is higher so as to decrease its gradient. The gradient is lower than the gradient of the magnetization curve (12) near the normal initial permeability thereby decreasing the secondary output induced in the secondary winding. Thus, the working current is deteriorated to deteriorate the overinput characteristic. As described above, the overinput characteristic is deteriorated after passing the large excitation current because the magnetization is remained at the point Br shown in FIG. 3. Thus, it is considered to be enough to use a magnetic core having lower Br. As one method, a magnetic core having a gap such as a separate type magnetic core is used, the magnetization curve shown by the chain line in FIG. 3 is given to decrease the residual magnetization to the point (13) of B'r whereas the permeability is remarkably decreased. FIG. 4 is a well-known graph showing the condition of the decrease of the permeability. When a gap having a ratio of 1/10,000 to the magnetic length of the magnetic core is formed, that is a gap of 10.mu. is formed in the magnetic core of a diameter of 3 cm (magnetic length of about 10 cm), the apparent permeability is decreased to 1/10 (10,000) for the magnetic core having apermeability of 100,000. It is difficult to obtain large output in such case.
When the non-leak balanced current is passed through the primary conductor of the residual current transformer, the secondary output may not be generated, however a slight noise output is generated in the practical operation.
The residual output percent is given by dividing the noise output by the working output. In such case, the balanced current is three times of the rated current (less than 20 A of rated current in single phase); or two times of the rated current (greater than 20 A of rated current in single phase); or several times depending upon the condition of the lines and the standard in a manufacturing company.
The noise output is depending upon the relative relation between the residual current transformer and the primary conductor. The magnetic core is rotated around the primary conductor. The maximum output is given as the unbalanced characteristic that is, the residual output.
In accordance with the normal process, these various characteristics are mutually affected in the same magnetic core to have the complicated relations and the design of the magnetic core is severely limited disadvantageously and the complicated process for preparing the magnetic core is required disadvantageously.